The present invention relates to semiconductor devices, and more particularly, field effect transistors, bipolar transistors, diodes and the like that are required to have a high breakdown voltage.
A MESFET (metal-semiconductor field effect transistor) is one of semiconductor devices in which compound semiconductors such as gallium arsenide (GaAs), indium phosphide (InP), and gallium nitride (GaN) are used. A MESFET has a Schottky junction in which a metal gate electrode is formed directly on a semiconductor layer, and the size of a depletion layer formed in the semiconductor layer due to the Schottky junction is made to change according to a voltage applied to the gate electrode, thereby controlling a current flowing through the channel layer.
One type of MESFET is a HEMT (high electron mobility transistor) in which a heterojunction structure is used. HEMTs, which are capable of operating at high speed, are widely applied, e.g., to low-noise amplifiers for receiving satellite broadcasting, switching devices and power devices for cellular telephones, and the like.
In order to use MESFETs, including HEMTs, as switching devices, power devices or the like, it is necessary to increase the gate-drain breakdown voltage thereof, and therefore it is preferable that in the MESFETs the semiconductor layer on which the gate electrode is formed be made of an undoped semiconductor that has a wider bandgap than the channel layer.
FIG. 9 illustrates a cross-sectional structure of a conventional GaAs-MESFET. As shown in FIG. 9, a channel layer 102 made of an n-type semiconductor is formed on a semi-insulating GaAs substrate 101, and an undoped aluminum gallium arsenide (AlGaAs) layer 103 is formed on the channel layer 102. A gate electrode 108 is formed on the AlGaAs layer 103, and a source electrode 106 and a drain electrode 107 are formed with the gate electrode 108 interposed therebetween. The source electrode 106 and the drain electrode 107 are respectively formed on a source region 104 and a drain region 105, which are high carrier concentration regions formed on the GaAs substrate 101. On the surface of the MESFET thus formed, a protective film 109, which is made of an insulating film such as a silicon oxide film (SiO2), is provided.
The protective film 109 is provided to prevent the creation of dangling bonds and oxidation-induced surface states on the semiconductor layer surface. In recent years, a BCN film made of boron, carbon, and nitrogen has also been used as a protective film to reduce the dielectric constant of the protective film (see Japanese Laid-Open Publication No. 2003-115487, for example.)
However, crystal defects or the like, occurring during a crystal growth step or other fabrication process steps, are present on the surface of the undoped AlGaAs layer 103, causing the resistivity of the surface to be nonuniform. In a case where the surface of the undoped AlGaAs layer 103 has a nonuniform resistivity, if a high-voltage reverse bias is applied between the gate electrode 108 and the drain electrode 107, the electric field is locally concentrated in a region due to the nonuniform resistivity. This results in a problem in that the breakdown voltage of the semiconductor device drops significantly below a design value.
Similar problems arise not only in field effect transistors but also hetero-bipolar transistors (HBTs), Schottky diodes and the like.
Such significant decrease in breakdown voltage is likely to occur in semiconductor devices in which compound semiconductors, such as GaAs, InP, and GaN, are used. Since the semiconductor material of a compound semiconductor contains two or more elements, a deviation from the stoichiometric composition is likely to occur on a semiconductor layer surface, such that the resistivity on that semiconductor layer surface easily becomes nonuniform.